The long rumored Martian Manhunter, J'onn J'onzz, is joining the fight as the fifth DLC character for the fighting game. Manhunter will not be covered by the first season pass, which was good for the first 4 and a Flashpoint skin pack. Neather Realm Studios could always offer another season pass meaning even more DLC characters. NRS has been using the release of characters as a point to add major patches to the game.
For his part MM has the stretchy moves of Dhalsim but moves quicker and is overal shown to be more offesnive, something Injustice seems to favor. Sadly no Shang Sung style character morphing.
Also shown in the trailer was a John Stewart skin for Green Lantern (Hal Jordan). This new skin also sounds like a new voice.